Coma corrector and back focus

[JayTee]

[mention]notFritzArgelander[/mention] ,

This is an excellent treatment of the subject with a clear and succinct answer. To me, this info is all new and magical. I have an 8" f/7.5 Newt with 1.25" focuser. Based on the above info, I do not have an eyepiece that should show coma. As it turns out, I have never seen coma through this scope.

Thanks again for making this very clear. I would like to turn your post into a stickie so it doesn't get lost in the shuffle/background.

Cheers,
JT

[JayTee]

WHOA! Quite a document! Thanks!

That's why I call it the Bible of Telescope Optics!

Cheers,
JT

[turboscrew]

....

I had written a lot more about this article, but the more I wrote the more I realized that I didn't fully understand all that was being said. @notFritzArgelander, or anyone else, if you'd like to weigh-in to interpret/decipher, that would be greatly appreciated.

Cheers,
JT

PS, I think I just opened a can of worms! jt

Let's discuss the blue line first. Coma increases with distance from the center of field. So the blue curve in the graph gives the distance in the image from the center of field where there is 3 arc minutes of visual coma as a function of the f ratio of the primary (the x axis).

The eye notices coma if it is more than 5 arc minutes when viewed from the eye. So the red line corresponds to when the eye can resolve the coma and it becomes obnoxious.

I use the ideas behind the graph to decide whether to use a coma corrector or not with an eyepiece. To use the graph (or the equations behind it) one needs to know the field stop of the eyepiece and the f ratio of the scope. Specifically I use a simpler method of knowing the FS (field stop) of the eyepiece and computing the following quantity:

Newtonian_limit = square-root(2*FS)

If that number is larger than the f ratio of the scope a coma corrector is needed. The need for a coma corrector depends on the eyepiece being used. There is no magical f ratio that is good for all eyepieces / magnifications. If your eyepiece DOESN'T have a field stop then use the size of the barrel. If you can't find the mfr's FS specification you can estimate the FS from

FS = (fl*AFOV/57.3)*(26/27.3)

Here's some worked examples:

Televue Panoptic 41mm fl, FS = 46mm. Newtonian_limit = square-root(2*FS) = 9.6. So this could be used without a coma corrector in a Newtonian at f9.6 or slower.

Televue Panoptic 19mm fl, FS = 21.30. Newtonian_limit = square-root(2*FS) = 6.5. So this could be used without a coma corrector in a Newtonian at f6.5 or slower.

Televue Plössl 11mm fl, FS = 7mm. Newtonian_limit = square-root(2*FS) = 4.3. So I could use this without a coma corrector in my f5 Z12 but things would get hairy around the edges in my f4 AT 8".

Higher magnification eyepieces have smaller field stops that block out the part of the FOV that has coma.

I no longer have the AT8 f4, but when I did a coma corrector was essential. I favored the TV Paracorr since it was so very easy to use. It has a built in helical focuser with markings so that if you know the setting for your eyepiece you can just dial it in and use the focuser on your scope. TV provides the settings in its specification page for eyepieces.

If you prefer a different brand of eyepiece, no worry. Just experiment.

The above formulas are pretty conservative. I find I could get by ignoring the coma with a 24mm Panoptic in my f5 Z12. At f4 I never am without the coma corrector.

PS If you need references to how/where I got these formulas, that's another post.

I can probably pass a good part of the bad weathers we have now by trying to figure out how you got the newtonian limit formula.

[notFritzArgelander]

From the Bible of Telescope Optics (called Amateur Telescope Optics). This is what happens when you want a more in-depth answer. It's almost more than I can digest. https://www.telescope-optics.net/newton ... ations.htm


this is just the first graphic of many on this subject.

If you read the linked page on this aberration, coma, (all the way to the bottom) you will see that the f/8 number is mostly correct.

I had written a lot more about this article, but the more I wrote the more I realized that I didn't fully understand all that was being said. @notFritzArgelander, or anyone else, if you'd like to weigh-in to interpret/decipher, that would be greatly appreciated.

Cheers,
JT

PS, I think I just opened a can of worms! jt

WHOA! Quite a document! Thanks!

It’s complicated so the document has to be big.

The main idea is that coma is a function of the distance from the center of the focal plane. The eyepiece magnifies the focal plane for inspection. If magnification is large enough you’ll be only looking at the part of the focal plane where coma is small. High magnification means small field stop.

[turboscrew]

From the Bible of Telescope Optics (called Amateur Telescope Optics). This is what happens when you want a more in-depth answer. It's almost more than I can digest. https://www.telescope-optics.net/newton ... ations.htm


this is just the first graphic of many on this subject.

If you read the linked page on this aberration, coma, (all the way to the bottom) you will see that the f/8 number is mostly correct.

I had written a lot more about this article, but the more I wrote the more I realized that I didn't fully understand all that was being said. @notFritzArgelander, or anyone else, if you'd like to weigh-in to interpret/decipher, that would be greatly appreciated.

Cheers,
JT

PS, I think I just opened a can of worms! jt

WHOA! Quite a document! Thanks!

It’s complicated so the document has to be big.

The main idea is that coma is a function of the distance from the center of the focal plane. The eyepiece magnifies the focal plane for inspection. If magnification is large enough you’ll be only looking at the part of the focal plane where coma is small. High magnification means small field stop.

I almost got that. Just wondered whether the magnification also magnifies the effect of the smaller coma in the region closer to optical axis...

[notFritzArgelander]

WHOA! Quite a document! Thanks!

It’s complicated so the document has to be big.

The main idea is that coma is a function of the distance from the center of the focal plane. The eyepiece magnifies the focal plane for inspection. If magnification is large enough you’ll be only looking at the part of the focal plane where coma is small. High magnification means small field stop.

I almost got that. Just wondered whether the magnification also magnifies the effect of the smaller coma in the region closer to optical axis...

Yes, but the formulas take that into account.

[turboscrew]

I got cheap and ordered Explore Scientific HR2 coma corrector. I hope I don't regret...

[notFritzArgelander]

I got cheap and ordered Explore Scientific HR2 coma corrector. I hope I don't regret...

I hope you are happy with it! It's designed by Harrie Rutten so it has excellent design pedigree on the optics.

Let us know how it works for you?

[notFritzArgelander]

@notFritzArgelander ,

This is an excellent treatment of the subject with a clear and succinct answer. To me, this info is all new and magical. I have an 8" f/7.5 Newt with 1.25" focuser. Based on the above info, I do not have an eyepiece that should show coma. As it turns out, I have never seen coma through this scope.

Thanks again for making this very clear. I would like to turn your post into a stickie so it doesn't get lost in the shuffle/background.

Cheers,
JT

Perhaps I could rework the post so it is clearer and more coherent, easier to understand?

[turboscrew]

I got cheap and ordered Explore Scientific HR2 coma corrector. I hope I don't regret...

I hope you are happy with it! It's designed by Harrie Rutten so it has excellent design pedigree on the optics.

Let us know how it works for you?

With these weathers, it may take a while, before I have a chance to try it.

[JayTee]

Perhaps I could rework the post so it is clearer and more coherent, easier to understand?

That would be wonderful for you to do that. I will make it an article in the articles section.

Thank you so much,
JT

[notFritzArgelander]

Perhaps I could rework the post so it is clearer and more coherent, easier to understand?

That would be wonderful for you to do that. I will make it an article in the articles section.

Thank you so much,
JT

OK will do.

[OzEclipse]

I can offer you a subjective assessment.

I have an 18inch f5.5 newtonian. I mostly use Naglers 31, 17, 12 which are a good match to a fast newt. I don't use a coma corrector and don't notice the coma. You might notice photographically but not visually.

The scopes previous owner is Matt (AKA Rainmaker). He also didn't use a coma corrector during his ownership of the scope.

F8 may be for photographic applications. But the "mileage may vary" with the type of eyepiece and it's degree of correction.

cheers

Joe